Magnetic disk files provide primary data storage systems for computer systems. The data is recorded in concentric tracks of a magnetic disk in the form of magnetic transitions. The disks are mounted on a spindle and the information is accessed by an actuator which moves a magnetic transducer radially over the surface of the disk and aligns the transducer with the concentric tracks. The disk and spindle are mounted for rotation on a support shaft and the disks are rotated at high speeds by means of an electric motor.
Important requirements for magnetic disk files are quick access to data together with high data rates. A key to both is a high rotational speed. On average, it takes half a revolution of the disk for the desired data to reach the transducer after the actuator has positioned the transducer at the desired track. Thus, the higher the speed the disk rotates, the quicker the desired data can be accessed. Similarly, faster rotation of a disk causes more data to pass the transducer, increasing the data rate at the transducer.
Fluid dynamic bearings may be used to achieve faster rotation speeds. In these systems, a lubricating fluid, such as oil, functions as the actual bearing surface between the stationary support shaft and the rotating spindle.
Typically, the lubricating fluid is maintained in the bearing areas by means of surface tension seals and a careful balance between the surface tension of the lubricant, the relative pressures of the air and the lubricant, and the size and structure of the clearances at the edges of the bearings.
For magnetic disk drive spindles, two fluid dynamic bearings are typically required with one at the top and one at the bottom of the spindle, respectively The bearings are held in place by surface tension, but are sometimes subject to rupturing by entrapped air and are therefore vented to avoid rupturing the oil film. The entrapped air can occur in low pressure and/or high temperature conditions. When the support shaft for the spindle is hollow and is press fitted into a hole in a disk file base plate, the entrapped air then vented out through the press fit hole in the base plate.
Referring to FIG. 1, an example of the described fluid dynamic spindle bearing 10 of the prior art is illustrated with two journal bearings 12 and 14. A spindle bushing 16 is mounted on a support shaft 16 with a small clearance. The small clearance is filled with a fluid lubricant, such as oil, through which fluid dynamic pressure is generated as the spindle bushing is rotated. The lubricating fluid is maintained in the bearing journals 12 and 14 by means of surface tension seals employing a careful balance between the surface tension of the lubricant, the relative pressures of the air and the lubricant, and the size and structure of the clearances at the edges of the bearings.
Two fluid dynamic bearings 12 and 14 are employed toward the top and bottom of the magnetic disk drive spindle, respectively. A recess 20 is provided defining or separating the bearing journals 12 and 14. The illustrated bearings 12 and 14 are normally coupled. If coupled, the bearings are sometimes decoupled by entrapped air, risking rupturing the oil film surface tension seals. The rupturing can occur due to expansion of the entrapped air due to low external ambient pressure and/or high temperature conditions. The bearings 12 and 14 may include a venting hole to the hollow center opening 23 in the support shaft 18 to reduce the likelihood of rupturing the oil film surface tension.
Alternatively, the bearings may be initially decoupled, forming surface tension seals at the top and bottom of each bearing 12 and 14. In the case of decoupled bearings, air is already present between the two radial bearings, but sometimes additional air can be entrapped during assembly or oil charging or even during operation. Thus, venting is often necessary to avoid rupturing the oil film. The venting hole 22 is therefore provided to vent the entrapped air through the hollow center opening 23 in the support shaft 18.
The support shaft 18 is press fit into an opening in a base plate 25. The press fit provides an external vent for the center opening 23 of the support shaft. Press fitting of the support shaft into the press fit hole in the base plate can be an expensive arrangement requiring accurate machining of the hole.
A more easily manufacturable alternative may be a screw mount through a flat base plate into the bottom of the support shaft 18. Unfortunately, the venting function is impeded by the presence of the screw and the contact between support shaft 18 and base plate 25 even if venting hole 22 is provided. A hollow screw could be employed, but is not suitable from a cost perspective.